The present invention relates to the measurement of a current waveform, particularly relates to current measurement technique and a current measurement apparatus for measuring high frequency current that flows in a transmission line of an electric circuit which is a load of small impedance.
Heretofore, to measure a current waveform, a method of inserting a resistor of approximately 10Ω in series with an electric circuit and monitoring potential difference between both ends of the resistor by a voltage probe has been adopted.
The defects of this method are as follows.
(1) As the resistor is inserted, the device under a test (DUT) is processed (destructive measurement).
(2) As the proper impedance of the device under the test is varied by the insertion of the resistor, the proper current cannot be measured (electric invasive measurement).
For another method of measuring a current waveform, there is a current probe. In this current probe, current is measured by surrounding the transmission line with a series of ring transformer, applying a magnetic field generated based upon current to this transformer and monitoring induced voltage hereby generated. The defects of this method are as follows.
(3) Lead wire is required to be added to the device under a test and is required to be through the transformer (destructive measurement).
(4) The proper impedance of the device under the test is varied by the impedance (approximately 20 nH because the length is approximately 20 mm) of the inserted lead wire and the proper current cannot be measured (electric invasive measurement).
(5) The bandwidth of the current probe is approximately 1.5 GHz and is small (low-time resolution).
A load of small impedance according to the invention is equivalent to a writing element of a magnetic head of HDD for example. The impedance is 10Ω20 nH or less and as an operating frequency is increased, the impedance has a tendency to further lower. In the above-mentioned conventional measurement technique, as impedance equivalent to the load or larger than the load is inserted into the device under the test, a current waveform far from the proper current is measured.
For a document related to the technique of this type, Japanese Laid-Open No. 2000-46891 can be given.